The Shape of Code

Before the internet took off in the 1990s, COBOL was the most popular language, measured in lines of code in production use. People who program in Cobol often have a strong business focus, and don’t hang out on sites used aggregated by surveys of programming language use; use of the language is almost completely invisible to those outside the traditional data processing community. So who knows how popular Cobol is today.

Despite the enormous quantity of Cobol code that has been written, very little Cobol source is publicly available (Open source or otherwise; the NIST compiler validation suite is not representative). The reason for the sparsity of source code is that Cobol programs are used to process business data, and the code is useless without the appropriate data (even with the data, the output is only likely to be of interest to a handful of people).

Program and function/method size (in LOC) are basic units of source code measurement. Until open source happened, published papers containing these measurements were based on small sample sizes and the languages covered was somewhat spotty. Cobol oriented research usually has a business orientation, rather than being programming oriented, and now there is a plentiful supply of source code written in non-Cobol languages.

I recently discovered appendix B of 1st Lt Richard E. Boone’s Master’s thesis An investigation into the use of software product metrics for COBOL systems (it’s post Rome period). Several days/awk scripts and editor macros later, LOC data for 178 programs containing 2,682 paragraphs containing 53,255 statements is now online (code+data).

A note on terminology: Cobol functions/methods are called paragraphs.

A paragraph is created by attaching a label to the first statement of the paragraph (there are no variables local to a paragraph; all variables are global). The statement PERFORM NAME-OF-PARAGRAPH ‘calls’ the paragraph labelled by NAME-OF-PARAGRAPH, somewhat like gosub number in BASIC.

It is possible to specify a sequence of paragraphs to be executed, in a PERFORM statement. The statement PERFORM NAME-OF-P1 THRU NAME-OF-P99 causes all paragraphs appearing textually in the code between the start of paragraph NAME-1 and the end of paragraph NAME-99 to be executed.

As far as I can tell, Boone’s measurements are based on individual paragraphs, not any sequences of paragraphs that are PERFORMed (it is likely that some labelled paragraphs are never PERFORMed in isolation).

Appendix B lists for each program: the paragraphs it contains, and for each paragraph the number of statements, McCabe’s complexity, maximum nesting, and Henry and Kafura’s Information flow metric

There are, based on naming, many EXIT paragraphs (711 or 26%); these are single statement paragraphs containing the statement EXIT. When encountered as the last paragraph of a PERFORM THU statement, the EXIT effectively acts like a procedure return statement; in other contexts, the EXIT statement acts like a continue statement.

In the following code the developer could have written PERFORM PARA-1 THRU PARA-4, but if a related paragraph was added between PARA-4 and PARA-END_EXIT all PERFORMs explicitly referencing PARA-4 would need to be checked to see if they needed updating to the new last paragraph.

START.
   PERFORM PARA-1 THRU PARA-END-EXIT.
 
PARA-1.
   DISPLAY 'PARA-1'.
 
PARA-2.
    DISPLAY 'PARA-2'.
 
PARA-3.
    DISPLAY 'PARA-3'.
 
P3-EXIT.
    EXIT.
 
PARA-4.
    DISPLAY 'PARA-4'.
 
PARA-END-EXIT.
    EXIT.

The plot below shows the number of paragraphs containing a given number of statements, the red dot shows the count with EXIT paragraphs are ignored (code+data):

How does this distribution compare with that seen in C and Java? The plot below shows the Cobol data (in black, with frequency scaled-up by 1,000) superimposed on the same counts for C and Java (C/Java code+data):

The distribution of statements per paragraph/function distribution for Cobol/C appears to be very similar, at least over the range 10-100. For less than 10-LOC the two languages have very different distributions. Is this behavior particular to the small number of Cobol programs measured? As always, more data is needed.

How many paragraphs does a Cobol program contain? The plot below shows programs ranked by the number of paragraphs they contain, including and excluding EXIT statements (code+data):

If you squint, it’s possible to imagine two distinct exponential declines, with the switch happening around the 100th program.

It’s tempting to draw some conclusions, but the sample size is too small.

Pointers to large quantities of Cobol source welcome.